In reply to Timmd:
He's a rescue cat and is incredibly needy and skittish. He doesn't go outside because other cats bully him (I've seen a pigeon bully him). Worse still, he expresses any kind of upset through the medium of shitting in surprising and difficult to reach, but unfortunately not difficult to smell, places. We love him very much.
Thanks for the opinions folks. Sounds like a mix of pet-friendly and contact sensors in rooms where he can get up high enough to set motion sensors off.